docs: def save(self, wfs, filename=None):
          '''Save a Werkzeug FileStorage object'''
          if self.basename and not filename:
              ext = extension(filename or wfs.filename)
              filename = '.'.join([self.basename(self._instance), ext])
          prefix = self.upload_to(self._instance) if callable(self.upload_to) else self.upload_to
          self.filename = self.fs.save(wfs, filename, prefix=prefix)
          return self.filename